The invention is a novel measuring instrument for a pressure gauge and a novel method for manufacturing a measuring instrument for a pressure gauge.
Measuring instruments for measuring pressure and methods for manufacturing them are known. The known measuring instrument can include a dial placed upon a front plate and an indicator placed upon an indicator pin. The unit comprising the measuring instrument, dial plate, and indicator can be arranged in a housing that carries a window in front of the dial plate. These elements together constitute a pressure gauge.
The known measuring instrument generally comprises a spring carrier, Bourdon tube, spring end-piece and an indicator unit. The measuring instrument together with the dial plate and an indicator are placed in a housing to form a pressure gauge. The indicator unit includes a bottom plate and gear segment. The gear segment includes a lever segment. The spring carrier generally comprises a cuboidal metal component with an integral bored device connector on one side and a channel on the other side. The device connector, which frequently embodies a threaded stem, connects the pressure gauge to a system so that measurement pressure (i.e., the pressure to be measured) is placed in communication with the channel. The one end of the Bourdon tube is inserted into the channel and soldered to the spring carrier. The other end of the Bourdon tube is soldered to the spring end-piece such that the other end is closed so as to be pressure-proof. The measurement pressure goes through the device connector into the curved Bourdon tube, which straightens to an extent that depends on the measurement pressure, so that its other end (and therefore the spring end-piece) is displaced. In the known system, the spring end-piece is flexibly joined to the lever section of the gear segment by means of a pull rod, so that the displacement of the spring end-piece is converted to rotation of the indicator shaft. The indicator on the dial indicates the prevailing measurement pressure.
In the course of manufacturing the known measuring instrument, the one end of the Bourdon tube is soldered to the spring carrier and the spring end-piece is soldered to the other end of the Bourdon tube. The indicator unit is assembled in parallel. The assembled indicator unit is then attached to the spring carrier. This is generally accomplished by bolting the bottom plate of the indicator unit to the spring carrier. Care must be taken that the Bourdon tube and the spring end-piece assume their target positions relative to the indicator unit as precisely as possible, especially relative to the axis of rotation of the gear segment, because the accuracy of the pressure measurement and the pressure indicator are affected by the accuracy with which these target positions are maintained. Once the indicator unit has been attached to the spring carrier, the pull rod is flexibly joined both to the spring end-piece and to the lever section of the gear segment so that displacement of the spring end-piece can be transmitted to the lever section.
The manufacture of this known measuring instrument is relatively labor-intensive and complex due to the steps described in the foregoing. In addition, there is no guarantee that the Bourdon tube and the spring end-piece will occupy their geometric target positions relative to the indicator unit. This can make it necessary to calibrate the measuring instrument, which can be more or less complex depending on the accuracy requirements for the instrument.
A known measuring instrument and a method for its manufacture are disclosed in the publication DE 23 54 473 C2. In this known measuring instrument, the spring carrier embodies either a metal piece that is bolted to the top plate into which piece the one end of the Bourdon tube is fixed air-tight, or it embodies a sheet-metal tab that either comprises a single piece with the top plate or is a separate piece that is bolted to said plate. If the spring carrier comprises a sheet-metal tab, the one end of the Bourdon tube is pushed onto the sheet-metal tab during the course of manufacture. In addition, a tube for feeding the measurement pressure is pushed into the one end of the Bourdon tube. The sheet-metal tab, the tube, and the one end of the Bourdon tube are then welded together. In this known measuring instrument, when the spring carrier is bolted to the plate of the indicator unit, the complexity of manufacturing and calibrating the measuring instrument is similar to that of the known measuring instrument described in the foregoing. If the sheet-metal tab constituting the spring carrier and the upper plate embody a single piece, it is true that joining the one end of the Bourdon tube to the spring carrier and manufacturing a fixed connection between this end of the Bourdon tube and the indicator unit comprise one step. However, the spring end-piece does not occupy a defined position with reference to the indicator unit with sufficient precision, so that the accuracy of the pressure indicator is poor or cannot be increased without more complex calibration.
The object of the invention is to further develop the generic measuring instrument such that the calibration complexity it requires is diminished. In other words, this means that the instrument should have comparatively high indicator accuracy even prior to calibration. Furthermore, manufacture of the measuring instrument is simplified.
Furthermore, the object of the invention is to further develop the generic manufacturing method such that the manufacture of the measuring instrument is simplified and such that a measuring instrument that indicates pressure with comparatively more accuracy is created with low manufacturing complexity.
The object in terms of the measuring instrument is achieved by a novel measuring instrument. This novel measuring instrument is distinguishable in that the spring end-piece constitutes a separate part that is attached to one of the two plates of the indicator unit prior to joining the Bourdon tube to the spring end-piece and detached from this plate after the Bourdon tube has been affixed to the spring end-piece. Since, the part constituting the spring end-piece is affixed to one of the two plates prior to being joined to the Bourdon tube, the spring end-piece assumes a defined position with respect to the elements of the indicator unit, especially with respect to the axis of rotation of the gear segment. During the course of manufacturing the plate to which the spring end-piece is attached, the position of the spring end-piece can be made to coincide with great accuracy with its target position. While the other end of the Bourdon tube is being rigidly joined to the spring end-piece, the latter is held almost exactly in its target position because it is affixed to the plate, and then maintains this target position even after it is separated from the plate. In this manner a measuring instrument is created that possesses comparatively high indicator accuracy even without calibration.
In one embodiment, the spring carrier is affixed to one of two plates. The spring carrier and the plate to which the spring carrier is affixed comprises a single metal sheet, which is bent in the shape of a cap with a U-shaped cross-section. Alternatively, the spring carrier can be a separate piece that is affixed to one of the two plates, wherein it embodies a sleeve and a connecting pin. The sleeve comprises a slit running in its longitudinal direction for accommodating one end of the Bourdon tube and a bore that opens into the interior space of the sleeve. The connecting pin is integral to the slit. In accordance with both of these aforesaid alternatives, the spring carrier is particularly compact and integrated into the indicator unit in a particularly useful manner. It is useful to employ these two alternative embodiments in a measuring instrument, even if the spring end-piece does not constitute a part attached to one of the two plates prior to being rigidly joined to the spring end-piece. Attaching the spring carrier to one of the two plates has the advantage that the spring carrier can be used as a device for positioning the one end of the Bourdon tube while the other end of the Bourdon tube is being joined to the spring end-piece and thus there is no need for a separate positioning apparatus.
The indicator unit comprises a preload spring that exerts a preload force on the gear segment. The indicator unit is rigidly joined to the spring end-piece which serves as a stop limit. The lever section of the gear segment is engaged with the stop limit due to the preload force of the preload spring. In this embodiment, the lever section of the gear segment always follows the movement (i.e., displacement) of the spring end-piece so that there is no need for a pull rod for creating the connection between the spring end-piece and the lever section of the gear segment. This connection without a pull rod can also be employed advantageously in measuring instruments in which the spring end-piece does not constitute a piece attached to one of the two plates prior to the Bourdon tube being joined to the spring end-piece.
With respect to the method, the object is achieved by a novel method. In accordance with the invented method, the spring end-piece is produced as a part securely joined to one of the two plates so that the spring end-piece is a component of the assembled indicator unit, so that the spring end-piece is brought into communication with the lever section prior to the other end of the Bourdon tube being rigidly joined to the spring end-piece. The other end of the Bourdon tube is rigidly joined to the spring end-piece while the latter is attached to the plate. Once the Bourdon tube has been joined to the spring end-piece, the spring end-piece is separated from the plate. This is a simple approach for creating a measuring instrument that has comparatively high indicator accuracy even without calibration, as has been explained previously.
In an alternative embodiment of the invented method, the spring carrier is integral to or affixed to one of the plates of the indicator unit before being joined to the end of the Bourdon tube, so that the spring carrier is a component of the assembled indicator unit. One end of the Bourdon tube is inserted into the spring carrier then the other end of the Bourdon tube is rigidly joined to the spring end-piece. In this case the position of the Bourdon tube is predetermined and fixed by the spring carrier while it is being joined to the spring end-piece. Therefore, no positioning apparatuses are required for this purpose.
Furthermore, in another alternative embodiment of the invented method, one end of the Bourdon tube is joined to the spring carrier at the same time that the other end of the Bourdon tube is joined to the spring end-piece. In this approach, the conventional steps of soldering the Bourdon tube to the spring carrier, soldering the Bourdon tube to the spring end-piece, and rigidly joining the indicator unit to the Bourdon tube are combined in a single manufacturing step.